Optical transceiver IC

ABSTRACT

A power management arrangement for low power optical transceiver such as those that may be integrated into a personal computer or server may periodically put itself into a power conservation or sleep mode which assures the transceiver is available upon wake-up.

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 12/347,029 filed Dec. 31, 2008 now U.S. Pat.No. 8,155,536.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to low power opticaltransceiver integrated circuits and, more particularly, to powermanagement for optical transceivers.

BACKGROUND INFORMATION

Optical communication networks have been implemented to enable increaseddata rates in links providing point to point communication. For example,optical communication links may be implemented in Synchronous OpticalNetwork/Synchronous Digital Hierarchy (SONET/SDH) and 10 GigabitEthernet systems. At a receiving end of such an optical communicationlink, a photodiode may generate a current in response an optical signalreceived from an optical transmission medium (e.g., fiber opticalcabling). A transimpedance amplifier (TIA) typically converts thecurrent generated by the photodiode into a voltage signal that is thenprocessed. For example, the voltage signal may be processed by clock anddata recovery circuitry to recover data transmitted in the opticalsignal.

As shown for example in FIG. 1, a typical optical transceiver integratedcircuit (IC) may use an analog approach to determine the input decisionlevel. Light 100 strikes a light sensitive device, such as a photodiode102 to produce an input current signal 104 to a TIA 106. The TIA 106 mayconvert the current signal 104 to an output voltage signal 108. Theoutput voltage signal 108 may be fed back through a simple RC filtercircuit comprising resistor 110 and capacitor 112 to derive an averagelevel of an input data stream 104 which can be served as the decisionlevel 114 for the TIA 106. This approach works as long as a continuousstream of 8/10B encoded DC-balanced data is applied to the input 104.8/10B refers to a line code that maps 8-bit symbols to 10-bit symbols toachieve DC-balance.

However, in a PC/Server like environment, power management is veryimportant from the system perspective. In order to save power, theoptical transceiver IC needs to get into certain sleep states whilethere is no input signal 104. That is, the input is not a continuousstream at all time anymore, and problems are raised for the typicalanalog approach.

While using the RC filter circuitry, the decision level 114 droops overtime due to the leakage current through the capacitor 112 (˜up to few μsrange). The change of the decision level 114 may lead to significanterrors depending on the capacitor value 112.

However, the decision level 114 needs to be correct as soon as thetransceiver recovers from a sleep state (which can be from few μs toseveral seconds) in order to function properly. Thus, in a PC/Serverenvironment where power management may be an issue a different approachwould be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and a better understanding of the present invention maybecome apparent from the following detailed description of arrangementsand example embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing arrangements and example embodiments ofthe invention, it should be clearly understood that the same is by wayof illustration and example only and the invention is not limitedthereto.

FIG. 1 is a circuit diagram for determining the decision levels for anoptical transceiver;

FIG. 2 is a circuit diagram for determining the decision levels for anoptical transceiver IC according to one embodiment of the invention; and

FIG. 3 is a circuit diagram for a signal detect circuit for an opticaltransceiver IC.

DETAILED DESCRIPTION

Described is power management arrangement for low power opticaltransceiver such as those that may be integrated into a personalcomputer or server that periodically puts itself into a powerconservation or sleep mode which assures the transceiver is availableupon wake-up.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

A photodiode as referred to herein relates to a device that provides anoutput current in response to light energy collected on a surface. Forexample, a photodiode may provide an output current in response tocharge collected at a photodiode gate. However, this is merely anexample of a photodiode and embodiments of the present invention are notlimited in this respect.

An amplifier as referred to herein relates to a device or circuit toconvert an input signal to an amplified output signal. For example, anamplifier may provide an amplified output signal having a magnitude thatis related to a magnitude of an input signal by an amplification gain.In another example, an amplifier may generate a voltage signal having amagnitude related by an amplification gain to a magnitude of either acurrent or voltage received as an input signal. However, these aremerely examples of an amplifier and embodiments of the present inventionare not limited in these respects.

A transimpedance amplifier (TIA) as referred to herein relates to adevice to convert an input current to an output voltage. For example, aTIA may convert an input current received from a photodiode to an outputvoltage that is substantially proportional to a magnitude of the inputcurrent. However, this is merely an example of a TIA and embodiments ofthe present invention are not limited in these respects.

As noted above, in the more common analog approach, the decision levelused by the TIA to determine the output level of the photodiode relieson a steady input signal which typically is not available in sleep mode.This change of the decision level may lead to significant errors atwake-up.

In order to solve the above issue, a digital loop is proposed todetermine the decision level for the optical transceiver. Referring toFIG. 2, there is shown a low power optical transceiver according to oneembodiment of the invention. As before, light 200 strikes a lightsensitive device, such as a photodiode 202 to produce an input currentsignal 204 to a TIA 206. Unlike before, the TIA 206 may providedifferential output voltage signals 208 and 209 based on the currentsignal 204 from the photodiode 202. A digital feedback loop may comprisea comparator which detects changes in the differential output voltagesignals 208 and 209. The output of the comparator 210 may be input intoa counter 212 which may change states depending on the light inputsignal 204. The digital output of the counter is fed to a digital toanalog converter (DAC) 214. The DAC outputs a current according to thedifference output from the comparator 210.

Since these are digital circuits, even in the sleep modes they remain atthe same states. Thus, the decision level 216 may be held infinitelyregardless the appearance of the input signal 204. Moreover, the looptime constant is determined by the counter clock 212 and can be adjusteddynamically to ensure the loop stability.

FIG. 3 is a circuit diagram for a signal detect circuit for an opticaltransceiver IC. This circuit may work in conjunction with the decisionlevel circuit of FIG. 2 to achieve a improved power management. Asshown, a light signal 200 may impinge on a light sensitive device, suchas a photodiode 202 which converts it to an electrical signal. Thiselectrical signal passes through a low pass filter comprising resistor300 and a capacitor 302 and into a buffer 304. According to embodiments,this circuit monitors an input signal and compares it to a referencecurrent 306 to adjust its output. Then the signal detect circuit outputgoes to a logic circuit to determine if a power saving mode should beentered into by the transceiver IC. In other words, the signal detectcircuit allows all the power-hungry circuit blocks be turned off or insleep states for lower power consumption over time if no light signal200 is present.

The above described embodiments may be advantageous, in manyapplications, for example in the optical universal serial bus (OpticalUSB or Converge I/O) standard. Since power management is a priority toPC/Server platforms, this invention is able to better handle the powerrequirements.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification and the claims. Rather, the scope of theinvention is to be determined entirely by the following claims, whichare to be construed in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. An apparatus, comprising: a light sensitivedevice to convert a light signal to an electrical signal; an amplifierto receive the electrical signal and an analog feedback signal, theamplifier further to produce a differential output including a pair ofdifferential output voltage signals, including the amplifier to amplifya difference between the electrical signal and the analog feedbacksignal, wherein the analog feedback signal represents an average levelof the differential output; a comparator to monitor a difference betweenthe differential output voltage signals; a counter to count up and downaccording to an output of the comparator; and a digital to analogconverter to convert a digital output of the counter into the analogfeedback signal.
 2. The apparatus as recited in claim 1 wherein theanalog feedback signal is held regardless of an appearance of the lightinput signal.
 3. The apparatus as recited in claim 1 wherein theamplifier comprises a transimpedance amplifier.
 4. The apparatus asrecited in claim 1 wherein the light sensitive device comprises aphotodiode.
 5. The apparatus as recited in claim 1, further comprising:a power conservation circuit connected to the light sensitive device. 6.The apparatus as recited in claim 5 wherein the power conservationcircuit comprises: a buffer to compare the output of the light sensitivedevice with a reference current; and a variable output voltagecontrolled by the buffer.
 7. The apparatus as recited in claim 6,further comprising: a low pass filter between the light sensitive deviceand the buffer.
 8. A method, comprising: detecting an input lightsignal; converting the light signal to an electrical signal; amplifyinga difference between the electrical signal and a decision level signalto produce a differential output including a pair of differential outputvoltage signals, wherein the decision level signal represents an averagelevel of the differential output; comparing the differential outputvoltage signals with a comparator; based on the comparing thedifferential output voltage signals, producing a digital counter signalfrom the output of the comparator; and converting the digital countersignal to an analog output to produce the decision level signal as afeedback for the amplifying the difference between the electrical signaland the decision level signal.
 9. The method as recited in claim 8wherein converting the light signal to the electrical signal comprisesconverting with a photodiode.
 10. The method as recited in claim 8wherein amplifying the difference between the electrical signal and thedecision level signal comprises amplifying with a transimpedanceamplifier.
 11. The method as recited in claim 8 further comprising:holding the decision level signal regardless of an appearance of theinput light signal.
 12. The method as recited in claim 8, furthercomprising: entering a power saving mode if no light input signal isdetected.
 13. The method according to claim 12, further comprising:comparing the electrical signal with a reference current; and varying asupply voltage according to the comparison to enter a power saving mode.14. The method as recited in claim 13, further comprising: passing theelectrical signal through a low pass filter prior to comparing.
 15. Anoptical transceiver system, comprising: a decision level detect circuit,comprising: a light sensitive device to convert a light signal to anelectrical signal; an amplifier to compare the electrical signal to adecision level signal, the amplifier to produce a differential outputbased on the compared electrical signal and decision level signal, thedifferential output including a pair of differential output voltagesignals, wherein the decision level signal represents an average levelof the differential output; a comparator to receive the differentialoutput; a counter to receive an output of the comparator; and a digitalto analog converter to convert a digital output of the counter into thedecision level signal; and a power saving circuit to put the decisionlevel circuit in a power saving mode when a light signal is not present.16. The system as recited in claim 15, wherein the power saving circuitfurther comprises: a variable voltage source to supply a voltage to thedecision level detect circuit.
 17. The system as recited in claim 15,wherein the decision level signal is held regardless of an appearance ofthe light input signal.
 18. The system as recited in claim 17, furthercomprising: a reference current to compare with the electrical signal.19. The system as recited in claim 15 wherein the optical transceiversystem comprises part of an optical universal serial bus (optical USB).